I have been reefing for for 20 years and have never seen a crab attack a coral or fish.
They might I guess if you do not feed them. Drop a pellet in their direction from time to time.

I agree they are completely fine in your tank.

Stow away number 1
http://img.tapatalk.com/464b5e07-36fb-3369.jpg

Second stow away
http://img.tapatalk.com/464b5e07-371f-1749.jpg

Help I'd?

I've had up to 4 in my display. A couple have since disappeared & I had to dispose of another several weeks ago. Caught him red handed with a piece of ricordia coral that I think he removed from the rock, frag plug & all. Pretty sure it was a Xanthidae, Actaeodes tomentosus. Had pointy piincers. The other known crab resident (Mithrax, Leptodius sp. Family Xanthidae) has scoops on his pincers & I've always only seen him/her scraping the live rock, never witnessed him damaging coral. He will take food scraps of any kind as well though. When my BTA was new & sickly, the crab would try to steal chunks of silverside I was trying to feed the nem.

As far as ID for yours, the first one looks like it may be an Emerald, so relatively useful & harmless in most tanks. Check to see if his pincers have scoops as opposed to being pointed. The scoops are better suited to scraping algae & coralline off the rockwork, so that's what they tend to do most of the time. Pointed pincers are better at tearing stuff up, like your small fish or coral. I've never seen one like in your second picture, so no clue on that one.

Do emerald crabs tend to just stay in one spot inside of the live rock or r they more active at night or day?... Cause both these crabs have never been seen walking around the rock day or night.

Also none of them seem to look green

Mine is not green either & think it's still considered an emerald crab. Not out much under the lights, except when feeding time. They can sense/smell when food hits the water, especially raw fleshy food like silversides or chunks of shrimp. Like I mentioned in my earlier post, an emerald would try to steal food from my BTA. Even my cleaner shrimp would grab stuff from out of it's tentacles. Now that the BTA is sticky & healthy, neither shrimp nor crab goes near it even when feeding silversides.

When I had two, each one seemed to hang out in opposite rock piles, territorial just like the fishes. They must have fought occasionally since both emeralds were missing appendages at various times. I'd definitely say more active at night. Get out your red light & you'll more than likely see it roaming around in the dark. Also depends on what other livestock you have in there & how long your system has been running. I see the crab out more often under the lights now that the tank's been up for years. Certain fishes don't take kindly to crabs & will take runs at them, depending on size & species. Survival instinct is strong.

Thanks guys I appreciate all the tips and starting to research ... The suspected emerald crab has pointy pinchers